Recently, the demand for cellular phone is mounting and the digitizing trend is dominating for effective utilization of frequency, and smaller size and lower cost are highly requested, and in this respect, in particular, the conventional digital cellular phones have many problems to be solved. Referring now to a drawing, an example is explained about the transmitter of cellular phone for portable use of digital modulation in the above prior art.
FIG. 1 shows a block diagram of a transmitter of a digital cellular phone in prior art. In FIG. 1, a digital modulator 204 is connected to a power amplifier 203, and is connected to a transmission antenna 201 through an isolator 202.
In the transmitter of the digital cellular phone of the prior art thus constituted, its operation is described below. A high frequency signal modulated by the digital modulator 204 is amplified by the power amplifier 203, and is transmitted from the antenna 201 through the isolator 202.
In such constitution of the prior art, however, in the cellular phone using digital modulation of quarter-pi-shifted QPSK, CDMA or the like, in order to control the adjacent channel leakage power which is unwanted radiation included in the signal sent out from the antenna 201 below the value specified in Radio Wave Law, it is required to keep constant the output load impedance of the power amplifier 203. The isolator 202 is designed to transmit the signal almost without attenuation in one direction, and absorb the opposite direction signal, and thereby it acts so as not to reflect the impedance change of the antenna 201 to the power amplifier 203. In the transmitter of the conventional digital cellular phone, to prevent increase of unwanted radiation due to fluctuation of load impedance of the high frequency power amplifier, it needed an expensive and large isolator with a large transmission loss.